The present invention relates to a method for machining stainless steel components; and more particularly, to a method for machining a stainless steel exhaust manifold for a multi-cylinder combustion engine.
As automotive combustion engine technology increases the efficiency in which the fuel is burned by the combustion engines, the exhaust temperatures in such combustion engines is increasing with the increase in efficiency.
Prior to the mid-1970's, the automotive industry traditionally used gray iron as the casting alloy for exhaust manifolds because it was low cost and it had a fairly high degree of heat resistance. This alloy was sufficient because the exhaust temperatures seldom exceeded 650° C. In the mid-70's, changes in the federal emission standards caused the combustion parameters to become more efficient, which resulted in a rise in exhaust temperature over 100° C. This rise in exhaust temperature sparked the development of ductile (or nodular) iron where the graphite is a spherical shape rather than the usual flake shape of gray iron. With the introduction of air injection reaction (AIR) systems into the exhaust manifolds, the exhaust temperatures began rising higher than 760° C.; and, further, the internal manifold atmosphere became strongly oxidizing. In response, the silicon content of the nodular iron was increased from 2.5 percent to 4.0–6.0 percent for oxidation resistance. This increased silicon percentage also increased the temperature at which ferrite to austenite transformation occurred from 800° C. to approximately 870° C. In response, molybdenum was added to the nodular iron in quantities of up to two percent (producing Si—Mo iron) during the early 1980's to further increase temperature resistance.
In the mid to late 1990's and beyond, as the exhaust temperatures for some commercially-produced combustion engines rose above 950° C. to approximately 1,030° C., new stainless steel alloys have been developed for the manifolds that may include, for example, the following chemical composition:
ElementComposition, Weight PercentageCarbon<0.6%Silicon<1.8%Manganese<1.0%Chromium24.0 to 27.0%Molybdenum0.50% Max.Nickel12.0 to 15%Phosphorus0.04%Nitrogen0.08 to 0.40%Niobium2.0%Other Residual Elements0.50% Max.IronBalance
Such new stainless steel materials contain basic elements and chemistry that require unique methods of metal removal (machining) not experienced in the past. Such stainless steel manifolds contain basic elements that are not compatible with the standard machining practices, nor are they compatible with high volume machining. For example, such stainless steel exhaust manifolds contain relatively high percentages of chromium and nickel. Alloys with high percentages of these elements in the machining industry are considered not to be compatible with the conventional high volume machining methods. Additionally, sulfur, which was typically added to improve machinability, is no longer used due to environmental concerns (or is used in very low percentages)—further increasing the difficulty in machining such materials.
Further, because this new stainless steel composition is difficult to cast into thin sections using the traditional gravity casting methods, the manifolds casted with these new stainless steel compositions are casted using sand casting methods. The sand casting results in silica granules being impregnated into the stainless steel material. The silica is highly abrasive and decreases tool life. The sand scale may be as deep as 0.060 inches before the parent material is encountered.